Method and device for digital compensation of dynamic distortion in high-speed transmitters

ABSTRACT

A device and method of operation for digital compensation of dynamic distortion. The transmitter device includes at least a digital-to-analog converter (DAC) connected to a lookup table (LUT), a first shift register, and a second shift register. The method includes iteratively adjusting the input values via the LUT to induce changes in the DAC output that compensate for dynamic distortion, which depends on precursors, current cursors, and postcursors. More specifically, the method includes producing and capturing average output values for each possible sequence of three symbols using the shift register and LUT configuration. Then, the LUT is updated with estimated values to induce desired output values that are adjusted to eliminate clipping. These steps are performed iteratively until one or more check conditions are satisfied. This method can also be combined with techniques such as equalization, eye modulation, and amplitude scaling to introduce desirable output signal characteristics.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 16/825,637 filed Mar. 20, 2020, which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention generally relates to communication systems andintegrated circuit (IC) devices. More specifically, the presentinvention provides for a device and method of operation for digitalcompensation of dynamic distortion in high-speed transmitters.

Over the last few decades, the use of communication networks hasexploded. In the early days of the Internet, popular applications werelimited to emails, bulletin boards, and mostly informational andtext-based web page surfing. The amount of data transferred by suchapplications was relatively small. Today, the Internet and mobileapplications demand a huge amount of bandwidth for transferring photo,video, music, and other multimedia files. For example, a socialnetworking platform can process more than 500 TB of data daily. Withsuch high demands on data storage and data transfer, existing datacommunication systems need to be improved to address these needs.

To address the need for high-speed data transfer, communication systemsrequire transceiver devices that can transmit and receive dataaccurately at high rates and low power consumption. However, thetransmitter and receiver components of such devices can be affected bynumerous impairments that degrade the data signal. Particularly,high-speed transmitters can suffer from impairments, such as distortion,limited bandwidth, insertion losses, reflections, and noise. In somecases, the transmitter suffers from dynamic distortion, which not onlydepends on current symbols being transmitted but also on previous andsubsequent symbols (precursors and postcursors, respectively) as well.

There have been many conventional types of methods and devices forcompensation of signal impairments in transmitters. Unfortunately, theyhave been inadequate to address dynamic distortion. Therefore, improvedcommunication systems with devices and methods for compensation ofdynamic distortion in high-speed transmitters are highly desired.

BRIEF SUMMARY OF THE INVENTION

The present invention generally relates to communication systems andintegrated circuit (IC) devices. More specifically, the presentinvention provides for a device and method of operation for digitalcompensation of dynamic distortion in high-speed transmitters. Merely byway of example, the present invention is applied to a 4-level pulseamplitude modulation (PAM4) transmitter. However, the present inventionhas a much broader range of applicability, such as other PAM-typetransmitters, amplitude modulation (AM) type transmitters, AMtransceivers, AM communication systems, and the like.

According to an example, the present invention provides for a method ofoperating a transmitter device having a digital-to-analog converter(DAC) configured to a lookup table (LUT), a first shift register, and asecond shift register. This method uses a digital compensation techniqueto address dynamic distortion, a type of distortion that affects atransmitted signal that is also dependent on previously transmittedsymbols (precursors) and subsequently transmitted symbols (postcursors).This method can also be combined with techniques such as equalization,eye modulation, and amplitude scaling to introduce desirablecharacteristics to the output signal.

In an example, the method includes producing and capturing initialtransmitter output values. The LUT can be filled with a set of startingvalues configured to produce a valid waveform at the output of the DAC.Then, an input signal pattern is sent through the DAC to produce anoutput waveform that is captured by a recording device (e.g., on-chipreceiver, oscilloscope, etc.). The LUT starting values can span the fullrange of the DAC output, and the signal pattern can be any random signalpattern.

In an example, the method includes processing the initial output valuesto update the values in the LUT. From the captured output waveform, theaverage output value is determined for each possible sequence of threesymbols at the input. These average output values can be sampled fromthe middle of the eye of the output waveform. A desired outputincorporating desired characteristics (e.g., equalization, eyemodulation, amplitude scaling, etc.) is determined for each possiblesequence, and the desired output is adjusted (i.e., shifted and scaled)to avoid clipping at the upper and lower ends. Afterwards, the LUTvalues are updated according to the adjusted desired output using aleast mean squared (LMS) algorithm. These updated LUT values representestimated compensation values that will take the DAC output from theinitial average output to the adjusted desired output.

In an example, the method includes iteratively producing and capturingaverage output values, and then iteratively updating the values in theLUT according to the previous steps until one or more check conditionsare satisfied. Such check conditions can include a target signal tonoise and distortion ratio (SDNR), requirements for desired signalcharacteristics, and the like. Those of ordinary skill in the art willrecognize other variations, modifications, and alternatives.

According to an example, the present invention provides for atransmitter device configured to perform the dynamic distortioncompensation techniques as described previously. The transmitter devicecan include at least a DAC coupled to an LUT configured with a firstshift register and a second shift register. These shift registers areconfigured to provide delayed copies of the input signal to address thetransmitted precursor, current cursor, and postcursor symbols discussedpreviously. The LUT adjusts the input values to the DAC to inducedesired changes (according to the method above) to the input signaluntil desired check conditions are met. This transmitter device can beincorporated into a transceiver device, a communication system, or thelike. Of course, there can be other variations, modifications, andalternatives.

Many benefits are recognized through various embodiments of the presentinvention. Such benefits include low-power compensation techniques fordynamic distortion at the transmitter output. These techniques can beimplemented in combination with other techniques to introduce desirablesignal characteristics, such as equalization, eye modulation, amplitudescaling, and the like. Further, the present invention provides for alow-complexity architecture to implement such techniques in atransmitter device, transceiver device, or communication system, and thelike. Other such benefits will be recognized by those of ordinary skillin the art.

The present invention achieves these benefits and others in the contextof known data transmission technologies. However, a furtherunderstanding of the nature and advantages of the present invention maybe realized by reference to the latter portions of the specification andattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is an example 4-level pulse amplitude modulation (PAM4) eyediagram showing the effects dynamic distortion.

FIG. 2 is an example PAM4 eye diagram showing the effects of dynamicdistortion compensation according to an example of the presentinvention.

FIG. 3A is a simplified block diagram illustrating a conventionaltransmitter device implemented as a digital-to-analog converter (DAC).

FIG. 3B is an example eye diagram illustrating corresponding to thetransmitter device shown in FIG. 3A.

FIG. 4 is simplified block diagram illustrating a transmitter deviceaccording to an example of the present invention.

FIGS. 5A and 5B are simplified flow diagrams illustrating a method fordigital compensation of dynamic distortion according to an example ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to communication systems andintegrated circuit (IC) devices. More specifically, the presentinvention provides for a device and method of operation for digitalcompensation of dynamic distortion in high-speed transmitters. Merely byway of example, the present invention is applied to a 4-level pulseamplitude modulation (PAM) transmitter. However, the present inventionhas a much broader range of applicability, such as other PAM-typetransmitters, quadrature amplitude modulation (QAM) transmitters,amplitude modulation (AM) type transmitters, AM transceivers, AMcommunication systems, and the like.

As explained above, high-speed transmitters can be affected by numerousimpairments such as distortion, limited bandwidth, insertion losses,reflections, and noise. In some cases, such as in large-signalsingle-ended transmitters, the distortion depends not only on the symbolbeing transmitted, but also on previously and subsequently transmittedsymbols (i.e., precursors and postcursors, respectively). We call thistype of distortion “dynamic distortion”, and we note that it isdifferent from static distortion, which depends only on the symbol beingtransmitted. Dynamic distortion is also different from linearimpairments, which may introduce linear dependence on precursors andpostcursors but will not introduce distortion. As a result, dynamicdistortion cannot be addressed with established techniques, such asstatic predistortion or equalization.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

FIG. 1 is an example 4-level pulse amplitude modulation (PAM4) eyediagram showing the effects dynamic distortion. An extreme case for aPAM4 transmitter is shown in eye diagram 100 where the falling edge ismuch slower than the rising edge, which results in the bottom PAM4 eyebeing closed.

FIG. 2 is an example PAM4 eye diagram showing the effects of dynamicdistortion compensation according to an example of the presentinvention. For comparison purposes, the transmit eye in eye diagram 200has much less dynamic distortion and is more likely to provide goodperformance for a data link.

As shown by the comparisons of FIGS. 1 and 2, it is highly desirable toimplement compensation techniques to address dynamic distortion intransmitter devices. This distortion can sometimes be addressed withanalog techniques at the cost of increased power consumption, but oftensuch analog techniques are not effective at very high frequencies. Thus,the present invention uses digital compensation techniques that will beeffective for high-speed transmitters operating at very highfrequencies.

In an example, the present invention provides a digital compensationmethod to remove dynamic distortion from the transmitter output. Thismethod can be further enhanced to introduce certain desirablecharacteristics into the output signal, such as precursor or postcursorequalization, eye modulation, amplitude scaling, or the like. Merely byway of example, this method assumes a PAM4 modulation scheme. However,this method can be applied to transmitters using any type of amplitudemodulation (AM) scheme, such as other types of PAM schemes, quadratureamplitude modulation (QAM), and the like. An example derivation of thedigital compensation method is discussed in further detail below.

FIG. 3A is a simplified block diagram illustrating a conventionaltransmitter device 301. Without loss of generality, let us assume thatthe transmitter 301 is implemented as a digital-to-analog converter(DAC) 310, which turns input digital codes x_(n) into an output waveformy(t). FIG. 3B is an eye diagram 302 corresponding to the output oftransmitter 301. This output waveform y(t) can then be sampled at timest_(n) in the middle of the eye to yield samples y_(n) (as shown in FIG.3B). Also, without loss of generality, let us assume that thetransmitter 301 is sending PAM4 signals.

Ideally, the output voltage y_(n) at time t_(n) should only be afunction of the input code x_(n):

y _(n) =y(x _(n))

In practice, the output voltage depends on multiple precursors andpostcursors:

y _(n) =y[x _(n−l) ,x _(n−l+1) , . . . x _(n) , . . . ,x _(n+k−1) ,x_(n+k)]+ε_(n)

Here, x_(n−l), x_(n−l+1), . . . x_(n−1) are precursors and x_(n+1), . .. , x_(n+k−1), x_(n+k) are postcursors, and ε_(n) is the residual error,which can be due to noise or other impairments not correlated with thesignal. Without loss of generality, let us assume that the outputvoltage is a function of a single precursor and a single postcursor:

y _(n) =y[x _(n−1) ,x _(n) ,x _(n+1)]+ε_(n)

To compensate for dynamic distortion, we need a method that removes thepart of the distortion that is dependent on x_(n−1) and x_(n+1) (i.e.,the precursor and postcursor components, respectively). According to anexample of the present invention, the method can involve calculating theaverage value of each possible sequence in the transmission signal.

For each possible sequence of three symbols at the input (X⁻¹, X₀, X₊₁),let us define y[X⁻¹,X₀,X₊₁] as the average value of y_(n) whenx_(n−1)=X⁻¹, x_(n)=X₀, and x_(n+1)=X₊₁. For example, suppose we have along continuous-time output y(t) that we sample in the middle of the eyeto obtain samples y_(n) (as shown in FIG. 3B). From the sampled outputy_(n), we can select just instances where three consecutive samplescorrespond to input values (X⁻¹, X₀, X₊₁) and for these instances we canaverage the cursor output y_(n) to calculate y[X⁻¹,X₀,X₊₁]. As anexample, in the case of PAM4 transmission with no equalization, each ofX⁻¹, X₀, and X₊₁ can take one of four possible values. Hence, there are4³=64 possible sequences, and we can define an average valuey[X⁻¹,X₀,X₊₁] for each of these 4³ sequences.

Using these average values, we need to change the output signal toremove undesirable characteristics (i.e., dynamic distortion) andintroduce other desirable characteristics (precursor or postcursorequalization, eye modulation, or amplitude scaling). Let us define thedesired output voltage values that have said desirable characteristicsas y[X⁻¹,X₀,X₊₁]. Preferably, we should generate {tilde over(y)}[X⁻¹,X₀,X₊₁] without modifying the analog circuitry since that wouldentail increased power and design complexity and the desired performancemay not even be achievable for very high data rates. Instead, we canadjust the input values to the DAC in such a way as to induce therequired changes on the transmit signal at the DAC output. This can bedone with a shift register and a look-up table (LUT), as shown in FIG.4.

FIG. 4 is a simplified block diagram illustrating a transmitter deviceaccording to an example of the present invention. As shown, device 400includes a DAC 410, an LUT 420, and shift registers 431, 432. The LUT420 is coupled to the DAC 410 and configured to receive an input signalalong with delayed input signals from the shift registers 431, 432.

In an example, the input is passed through the shift registers 431, 432to generate delayed copies of the signal (x_(n−1), x_(n), and x_(n+1)),which are then used to index the LUT 420. Through this indexing, the LUT420 is configured with values or codes that can compensate for dynamicdistortion and introduce said desirable characteristics. In a specificexample, for each possible input sequence (X⁻¹, X₀, X₊₁), the LUT storescodes that cancel the effects of dynamic distortion in the DAC 410.

In other examples, the transmitter device 400 can include additionalcircuit components configured before or after the LUT-DAC configuration.Additional circuit components may be connected between the LUT and theDAC as well. The circuit components can include various filters,amplifiers, mixers, and the like and combinations thereof. In suchcases, the functionality of the LUT-DAC configuration remains the same.These examples of the transmitter device 400 can also be configuredwithin a transceiver device or in a broader communication system or thelike. Those of ordinary skill in the art will recognize variations,modifications, and alternatives to the configuration shown in FIG. 4.

Although LUTs are used to apply equalization, eye modulation oramplitude scaling, examples of the present invention use the LUT toperform more powerful signal processing, including dynamic distortioncompensation.

For instance, linear equalization can be implemented as:

z[X ⁻¹ ,X ₀ ,X ₊₁]=a ⁻¹ ·X ⁻¹ +a ₀ −X ₀ +a ₁ ·X ₊₁

and eye modulation and amplitude scaling can be implemented as:

z[X ⁻¹ ,X ₀ ,X ₊₁]=c(X ₀)

These two equations can be combined into the following equation:

z[X ⁻¹ ,X ₀ ,X ₊₁]=a ⁻¹ ·c(X ⁻¹)+a ₀ ·c(X ₀)+a ₁ ·c(X ₊₁)

From this equation, we can see that linear equalization, eye modulation,and amplitude scaling introduce certain relationships between theelements of the LUT. For example, for eye modulation all z[X⁻¹,X₀,X₊₁]entries for a given X₀ value are identical. On the other hand, fordynamic distortion compensation, it is important that each LUT entry isindependently set.

Let us first calculate the desired output voltage {tilde over(y)}[X⁻¹,X₀,X₊₁] including the required equalization, eye modulation,and amplitude scaling, using an equation similar to the one above:

{tilde over (y)}[X ⁻¹ ,X ₀ ,X ₊₁]=a ⁻¹ c(X ⁻¹)+a ₀ ·c(X ₀)+a ₁ ·c(X ₊₁)

The desired output must be shifted and scaled to make use of theavailable range of the DAC without clipping. For example, assuming a7-bit DAC, no codes can be lower than 0 and no codes can be higher than127. Therefore, we need to identify the sequences most likely to clip.Assuming that the output can have both positive and negative values,clipping at the upper end is most likely to occur for the sequence thathas the main cursor (X₀) set to the top PAM4 level. In addition, thissequence has the maximum ratio {tilde over(y)}[X⁻¹,X₀,X₊₁]/y[X⁻¹,X₀,X₊₁]. Let us define y_(max)=y[X⁻¹,X₀,X₊₁] forthis sequence most likely to clip at the upper end.

Similarly, clipping at the lower end is most likely to occur for thesequence that has the main cursor (X₀) set to the bottom PAM4 level.This sequence also has the maximum ratio {tilde over(y)}[X⁻¹,X₀,X₊₁]/y[X⁻¹,X₀,X₊₁]. Let us define y_(min)=y[X⁻¹,X₀,X₊₁] forthis sequence most likely to clip at the lower end.

Once y_(max) and y_(min) are identified, the signal can be easilyshifted and scaled to fit the available range without clipping asfollows:

${\overset{\hat{}}{y}\left\lbrack {X_{- 1},X_{0},X_{+ 1}} \right\rbrack} = {{\frac{S \cdot {\overset{˜}{y}\left\lbrack {X_{- 1},X_{0},X_{+ 1}} \right\rbrack}}{{\max\left( {\overset{˜}{y}\left\lbrack {X_{- 1},X_{0},X_{+ 1}} \right\rbrack} \right)} - {\min\left( {\overset{˜}{y}\left\lbrack {X_{- 1},X_{0},X_{+ 1}} \right\rbrack} \right)}} \cdot \left( {y_{\max} - y_{\min}} \right)} + {\frac{1}{2} \cdot \left( {y_{\max} + y_{\min}} \right)}}$

where S is the desired scaling factor and ŷ[X⁻¹,X₀,X₊₁] is the desiredoutput voltage for each sequence, after scaling and shifting.

The next step is to estimate the required LUT entries z[X⁻¹,X₀,X₊₁] thattake the output from the initial {tilde over (y)}[X⁻¹,X₀,X₊₁] values tothe desired values ŷ[X⁻¹,X₀,X₊₁]. In an example, this can be done usingthe LMS algorithm as follows:

z _(k+1)[X ⁻¹ ,X ₀ ,X ₊₁]=z _(k)[X ₁ ,X ₀ ,X ₊₁]+μ·(ŷ[X ₁ ,X ₀ ,X ₊₁]− y[X ₁ ,X ₀ ,X ₊₁])

where μ is the convergence coefficient.

FIGS. 5A and 5B are simplified flow diagrams illustrating a method fordigital compensation of dynamic distortion according to an example ofthe present invention. As shown, flow diagram 501 of FIG. 5A connects toflow diagram 502 of FIG. 5B by the connector “A”. From the stepsdescribed above, we can now define the full procedure according to anexample of the present invention as follows:

-   -   1. Fill the LUT with a set of starting values configured to        produce valid waveforms at the output of the DAC (flow diagram        step 511). Generally, it is best to exercise the whole range of        the DAC at this step. For example, in the case of using a PAM4        coding scheme with a 7-bit DAC, we can fill the LUT with z[*,        X₀,*]=0/43/85/127.    -   2. Send an input signal pattern x_(n) through the DAC (flow        diagram step 512). This signal pattern can be any random signal        pattern (e.g., higher-order PRBS for PAM4).    -   3. Capture the output waveform with a recording device (flow        diagram step 513). This recording device can be an on-chip        receiver or test equipment, such as a digital scope, or the        like.    -   4. Determine the average output value y[X⁻¹, X₀, X₊₁] for each        possible sequence of three symbols at the input (flow diagram        step 514). These average output values can be sampled from the        middle of the eye of the output waveform.    -   5. Determine the desired output {tilde over (y)}[X⁻¹,X₀,X₊₁] for        each of these possible sequences (flow diagram step 515). The        desired output calculation can include techniques for linear        equalization, eye modulation, amplitude scaling, and the like        and combinations thereof.    -   6. Adjust the desired output to ŷ[X⁻¹,X₀,X₊₁] for all possible        sequences to eliminate the risk of clipping (flow diagram step        516). This adjustment includes determining the sequences most        likely to clip at the upper end and the lower end. Afterwards,        the adjusted desired output can be determined by the calculation        discussed previously.    -   7. Estimate the next iteration of LUT entries        z_(k+1)[X⁻¹,X₀,X₊₁] that will take the output from the initial        average output to the adjusted desired output (flow diagram step        517). As discussed above, the LUT entries can be estimated using        an LMS algorithm. However, this step may use other algorithms,        such as recursive least squares (RLS), Affine Projection (AP),        and the like.    -   8. Send the same input signal pattern x_(n) as before through        the DAC, as described in step 2 (flow diagram step 518).    -   9. Capture the output waveform using the recording device, as        described in step 3 (flow diagram step 519).    -   10. Check whether the output waveform meets one or more check        conditions (flow diagram step 520). These check conditions can        include whether the output waveform has low enough distortion        (e.g., by measuring signal to noise and distortion ratio (SNDR))        and additionally meets the requirements of equalization, eye        modulation and scaling set out in the beginning. Other check        conditions can include eye height (i.e., eye opening), bit error        rate (BER), level separation mismatch ratio (RLM), transmission        dispersion and eye closure quaternary (TDECQ), and the like. If        the desired check conditions are not satisfied, the process can        be repeated starting from step 4 (shown by flow diagram steps        521, 522, 523, and 524). In practice, iterations may be required        because dynamic distortion changes as the LUT is adapted.    -   11. Perform other steps, as desired (flow diagram step 530).

The above sequence of steps is used to perform a method for digitalcompensation of dynamic distortion according to one or more embodimentsof the present invention. Depending upon the embodiment, one or more ofthese steps can be combined, or removed, or other steps may be addedwithout departing from the scope of the claims herein. One of ordinaryskill in the art will recognize variations, modifications, andalternatives.

In an example, the method steps described above can be performed viahardware by a digital signal processor (DSP) electrically coupled to theLUT, the DAC, and the recording device. The DSP can also be programmedto perform these method steps via firmware stored in a non-volatilememory (NVM) device (e.g., read-only memory (ROM), erasable read-onlymemory (EPROM), flash memory, etc.) configured within the transmitterdevice or encompassing communication device. The DSP can also beimplemented directly in the recording device (e.g., in the receiver) andthe LUT updates can be communicated back to the transmitter device bymeans of a back channel, where available. Or, these methods steps canalso be performed by software, such as by a computing system with testequipment (e.g., digital scope) connected to the transmitter device toimplement the steps described previously. Of course, there can be othervariations, modifications, and alternatives.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A method of operating a transmitter device, themethod comprising: providing a lookup table (LUT) with a plurality ofstarting values; and iteratively performing, until one or more checkconditions are met, the following: sending an input signal patternthrough the LUT to a digital-to-analog converter (DAC) to produce anoutput waveform; capturing the output waveform using a recording device;determining a desired output value using the captured output waveformfor each possible sequence of three input symbols; and determining andstoring, in the LUT, an estimated LUT value for each possible sequenceof three input symbols associated with the desired output valuecorresponding to the same sequence of three input symbols; wherein thetransmitter includes a first shift register configured with the LUT toproduce a first delayed input signal pattern and a second shift registerconfigured with the LUT to produce a second delayed input signalpattern; and wherein the input signal pattern, the first delayed inputsignal pattern, and the second delayed input signal pattern are used toprovide each possible sequence of three input symbols used to determinethe desired output values and the estimated LUT values.
 2. The method ofclaim 1 wherein determining the desired output value includesdetermining an average output value using the captured output waveformfor each possible sequence of three input symbols and determining thedesired output value using the average output value for each possiblesequence of three input symbols.
 3. The method of claim 1 whereindetermining the desired output values and determining the estimated LUTvalues are performed by a digital signal processor (DSP); and whereinstoring the estimated LUT values in the LUT comprises communicating, bythe DSP, the estimated LUT values over a back channel configured betweenthe transmitter device and the recording device.
 4. The method of claim1 wherein determining the desired output values includes applying linearequalization, eye modulation, or amplitude scaling for each sequence ofthree input symbols; and wherein determining the estimated LUT valuesincludes applying a least mean squared (LMS) algorithm, a recursiveleast squares (RLS) algorithm, or an Affine Projection (AP) algorithm.5. The method of claim 1 further comprising, iteratively performing, thestep of adjusting the desired output values, including determining asequence of three input symbols that is most likely to clip at an upperend of the output waveform; determining a sequence of three inputsymbols that is most likely to clip at a lower end of the outputwaveform; and shifting and scaling the desired output values accordingto the sequences most likely to clip at the upper and lower ends.
 6. Themethod of claim 1 wherein the one or more check conditions includes atarget signal to noise and distortion ratio (SNDR), eye height, biterror rate (BER), level separation mismatch ratio (RLM), or transmissiondispersion and eye closure quaternary (TDECQ).
 7. A transmitter device,the device comprising: a lookup table (LUT) having a plurality of LUTentries configured to receive an input signal, the LUT being configuredto output an LUT output signal according to the plurality of LUTentries; a first shift register coupled to the LUT and configured toprovide a first delayed input signal to the LUT; a second shift registercoupled to the LUT and configured to provide a second delayed inputsignal to the LUT; a digital-to-analog converter (DAC) coupled to theLUT and configured to receive the LUT output signal and to output a DACoutput signal; a recording device coupled to the DAC and configured tocapture the DAC output signal; and a digital signal processor (DSP)coupled to the LUT, the DAC, and the recording device; wherein the DSPis configured to provide the LUT with a plurality of starting values inthe plurality of LUT entries; and configured to iteratively perform,until one or more check conditions are met, the following: sending aninput signal pattern as the input signal through the LUT to adigital-to-analog converter (DAC) to produce an output waveform in theDAC output signal; capturing the output waveform from the DAC outputsignal using the recording device; determining a desired output valueusing the captured output waveform for each possible sequence of threeinput symbols; and determining and storing, in the plurality of LUTentries, an estimated LUT value for each possible sequence of threeinput symbols associated with the desired output value corresponding tothe same sequence of three input symbols; wherein the input signal, thefirst delayed input signal, and the second delayed input signal are usedto provide each possible sequence of three input symbols used todetermine the desired output values and the estimated LUT values.
 8. Thedevice of claim 7 wherein the recording device includes an on-chipreceiver or a digital scope.
 9. The device of claim 7 wherein the DSP isconfigured to determine the desired output value by determining anaverage output value using the captured output waveform for eachpossible sequence of three input symbols and determining the desiredoutput value using the average output value for each possible sequenceof three input symbols.
 10. The device of claim 7 wherein the DSP isconfigured to determine the desired output values by applying linearequalization, eye modulation, or amplitude scaling for each sequence ofthree input symbols.
 11. The device of claim 7 wherein the DSP isconfigured to iteratively perform, the step of adjusting the desiredoutput values by determining a sequence of three input symbols that ismost likely to clip at an upper end of the output waveform; determininga sequence of three input symbols that is most likely to clip at a lowerend of the output waveform; and shifting and scaling the desired outputvalues according to the sequences most likely to clip at the upper andlower ends.
 12. The device of claim 7 wherein the DSP is configured todetermine the estimated LUT values by applying a least mean squared(LMS) algorithm, a recursive least squares (RLS) algorithm, or an AffineProjection (AP) algorithm.
 13. The device of claim 7 wherein the one ormore check conditions includes a target signal to noise and distortionratio (SNDR), eye height, bit error rate (BER), level separationmismatch ratio (RLM), or transmission dispersion and eye closurequaternary (TDECQ).
 14. A communication system, the system comprising: areceiver device; a communication channel coupled to the receiver device;and a transmitter device coupled to the communication channel, thetransmitter device comprising: a lookup table (LUT) having a pluralityof LUT entries configured to receive an input signal from the receiverdevice over the communication channel, the LUT being configured tooutput an LUT output signal according to the plurality of LUT entries; afirst shift register coupled to the LUT and configured to provide afirst delayed input signal to the LUT; a second shift register coupledto the LUT and configured to provide a second delayed input signal tothe LUT; a digital-to-analog converter (DAC) coupled to the LUT andconfigured to receive the LUT output signal and to output a DAC outputsignal; a recording device coupled to the DAC and configured to capturethe DAC output signal; and a digital signal processor (DSP) coupled tothe LUT, the DAC, and the recording device; wherein the DSP isconfigured to provide the LUT with a plurality of starting values in theplurality of LUT entries; and configured to iteratively perform, untilone or more check conditions are met, the following: sending an inputsignal pattern as the input signal through the LUT to adigital-to-analog converter (DAC) to produce an output waveform in theDAC output signal; capturing the output waveform from the DAC outputsignal using the recording device; determining a desired output valueusing the captured output waveform for each possible sequence of threeinput symbols; and determining and storing, in the plurality of LUTentries, an estimated LUT value for each possible sequence of threeinput symbols associated with the desired output value corresponding tothe same sequence of three input symbols; wherein the input signal, thefirst delayed input signal, and the second delayed input signal are usedto provide each possible sequence of three input symbols used todetermine the desired output values and the estimated LUT values. 15.The system of claim 14 wherein the recording device includes an on-chipreceiver or a digital scope.
 16. The system of claim 14 wherein the DSPis configured to determine the desired output value by determining anaverage output value using the captured output waveform for eachpossible sequence of three input symbols and determining the desiredoutput value using the average output value for each possible sequenceof three input symbols.
 17. The system of claim 14 wherein the DSP isconfigured to determine the desired output values by applying linearequalization, eye modulation, or amplitude scaling for each sequence ofthree input symbols.
 18. The system of claim 14 wherein the DSP isconfigured to iteratively perform, the step of adjusting the desiredoutput values by determining a sequence of three input symbols that ismost likely to clip at an upper end of the output waveform; determininga sequence of three input symbols that is most likely to clip at a lowerend of the output waveform; and shifting and scaling the desired outputvalues according to the sequences most likely to clip at the upper andlower ends.
 19. The system of claim 14 wherein the DSP is configured todetermine the estimated LUT values by applying a least mean squared(LMS) algorithm, a recursive least squares (RLS) algorithm, or an AffineProjection (AP) algorithm.
 20. The system of claim 14 wherein the one ormore check conditions includes a target signal to noise and distortionratio (SNDR), eye height, bit error rate (BER), level separationmismatch ratio (RLM), or transmission dispersion and eye closurequaternary (TDECQ).